1. Field of the Invention
The invention relates to a method for cleaning liquids containing impurities, in which an addition of adsorbents takes place, together with an apparatus for carrying out said method.
2. Related Art
It is known that in manufacturing processes impurities, for example dyestuffs, phosphatides and solids particles, have to be separated from liquid recyclates. For example, DE 31 23 462 A1 describes an apparatus for the electrostatic cleaning of dielectric liquids, in which the dielectric liquid is exposed to an electric field in a tank with an inflow opening for the contaminated dielectric and an outflow opening for the cleaned dielectric in the upper part of the tank, as well as an outlet opening for the impurities in the lower part above two systems of electrodes disposed therein. The separation of the very small particles which get into the dielectric liquid during electrochemical machining is barely possible with mechanical filters because of the low permeability of the filter cake formed. Electrostatic cleaning is particularly suitable for this.
In particular for the obtaining or processing of animal, vegetable or marine oils it is known; in general from DE 42 10 795 C1 that the liquids, in particular those for the production of foodstuffs, form a mixture together with solids particles. The solids particles and impurities can be inter alia undesirable phosphatides, soap residues and complex colloidal compounds, but also adsorbents added for performing a cleaning such as bleaching earths, activated carbon and silica. The adsorbents are mixed with the liquid for the cleaning. In so doing said adsorbents contact in a so-called bleaching stage inter alia dyestuffs such as carotene and chlorophyll, heavy metals, residues of phosphatides and soaps as well as peroxides. Due to the contact the substances from the oil or fat attach themselves to the adsorbents and can be removed from the liquid together with the latter.
The separation of the adsorbent loaded with the above-mentioned substances from the bleached oil takes place in the known process with the aid of pressure filters which operate according to the principle of precoat filtration. There are known for the filtering e.g. plate-and-frame presses, plate filters and tube filters, as well as crossflow filtration. It is a disadvantage with pressure filtration, however, that the filter cake formed inhibits strongly the flow of the liquid. In keeping with this, a considerable expenditure of energy is required for the pumping through of the liquid. Furthermore, the carrying capacity of the filters is rapidly exhausted, making frequent filter replacement or filter cleaning necessary.
Conversely, there is known from DE 43 44 828 A1 a method and an apparatus for cleaning liquids containing impurities in which the contaminated liquid is exposed to an electric field by means of mutually insulated electrodes, in which simultaneously the viscosity of the liquid to be cleaned is lowered by heating. With this method, although one part of the liquid can be withdrawn cleaned, a not inconsiderable part of the liquid nevertheless remains in the bleeding. It is furthermore disadvantageous that the layout and control of the electrodes are cumbersome.
The invention is based on the problem of specifying a method and an apparatus for cleaning liquids containing impurities, such as oils and fats, in which method/apparatus the separation takes place with relatively little equipment and low energy requirements.
The problem is solved for the method given in the preamble by the steps:
addition of adsorbents, which adsorb the impurities, to the liquid,
agglomeration of respectively several particles of adsorbent with the impurities adsorbed therein/thereon to form greater particles of adsorbent,
renewed suspension of the now greater particles of adsorbent in the liquid and
filtering of the liquid containing the suspended greater particles of adsorbent.
First of all, in the conventional manner, the relatively fine-grain particles of adsorbent adsorb the impurities that are present in the liquid as solids particles or dyestuff or other and are to be removed. When or while the particles of adsorbent have taken/been taking up the impurities, an agglomeration of the fine-grain particles of adsorbent is then carried out.
Preferably the suspension is first of all exposed to an electric field for a particular time, wherein the loaded particles are moved towards an electrode. There the particles agglomerate, are separated and are suspended in the liquid once again on disconnection of the electric field, wherein the particles of adsorbent exhibit as a result of the agglomeration an enlarged particle structure. In particular the particles have after the agglomeration a porous, loose structure.
The subsequent filtration, which is preferably carried out as a pressure filtration, can be executed more effectively by virtue of the enlarged particle structure, since the enlarged particles bestow a sufficient permeability for the liquid during the formation of the filter cake. Thus it is possible according to the invention to operate the filtration up to a considerably higher loading of the filter area.
If, as known from DE 43 44 828 A1, a fraction of the liquid which is substantially free of adsorbents and hence of solids particles is withdrawn simultaneously during the agglomeration of the solids, for example at an outlet disposed in the upper part of a tank, a part of the liquid to be separated has already been cleaned. There arises in the remaining liquid, on renewed suspension of the solids collected and attached first of all due to the electric field, a strongly concentrated suspension. Because of the porous structure of the agglomerated particles in the liquid to be cleaned during the subsequent pressure filtration the area of the precoat filters for example used can be reduced considerably.
The invention becomes particularly effective if it is used as a part step in a countercurrent bleaching process, such as is described in German patent application 196 20 695.2-41 published at a later date. In counter-current processes uncleaned oils are brought into contact with already partially loaded adsorbent in a stage and then in a second stage the oil partially cleaned in the first stage is brought into contact with fresh adsorbent. The adsorbent used in the second stage then serves as partially loaded adsorbent in the first stage. The end product of the second stage is then highly cleaned oil.
The concentrated suspension now obtained according to the invention, containing the already agglomerated and loaded particles of adsorbent, can now be treated in such a countercurrent process as the xe2x80x9cpartially loaded adsorbentxe2x80x9d and be introduced in the first stage together with the still uncleaned oil. The agglomerated particles of adsorbent can thus be used again with a saving on materials.
The use of bleaching earth as adsorbing agents is preferred for the cleaning of animal, vegetable or marine oils and/or fats, in particular those for the production of foodstuffs. The use of silica gels or activated carbon, for example, is however also possible.
Because the bleaching earth to be fed has a large fine grain content, for example with a grain spectrum of 5 to 10 xcexcm, the adsorptive power is increased due to the enlarged free surface of the fine-grain bleaching earth. It is no longer necessary, therefore; to use an excessive proportion of bleaching earth. The consumption of bleaching earth therefore falls with advantage. The amount of bleaching earth used can in the case of rape oil, for example, be reduced in absolute terms from the minimum 0.6% conventional to date to 0.3%. Furthermore, the fine grain bleaching earth is more favourable as regards procurement costs than the bleaching earth structured as a result of pretreatment which is used to date. For the grain size distribution has represented to date a compromise between a sufficiently good bleaching effect and a sufficiently good separability by conventional filter systems. However, since according to the invention fine-grain bleaching earth of high adsorptive power is used to start with and the latter is agglomerated to form more coarsely-grained particles, the suspension is optimally structured for the subsequent filtration.
A particularly economic pressure filtration is obtained if the solids content of the concentrated suspension is set to 10% to 30%, preferably 17% to 20%.
Preferably the method according to the invention is operated discontinuously, in which first of all a batch of liquid to be cleaned is exposed to an electric field and agglomerated, optionally a fraction of cleaned liquid is withdrawn, thereafter the electric field is disconnected and the particles of adsorbent are suspended in the (remaining) liquid, after which the concentrated suspension is filtered.
If the concentrated:suspension is likewise collected and only after this is fed to the pressure filtration stage, the pressure filtration can also be carried out economically, it being possible, in particular on the attainment of a particular filter cake loading, for the pressure filtration to be interrupted at any time in order to remove the filter cake.
If the agglomeration is carried out in two or more individual units, wherein the units thus load the pressure filtration staggered in time, the total process can take place continuously despite the batch-wise agglomeration.
In a further development it is preferred, particularly in order to improve existing process operations, that a part flow of the liquid containing solids particles be subjected to the agglomeration and the subsequent renewed suspension, wherein the concentrated suspension is fed to the unprocessed other part flow of the liquid and filtered together with it. The enlarged particle structures contained in the concentrated suspension increase in the following pressure filtration the permeability of the growing filter cake. A more economical cleaning can therefore also be achieved by means of a part treatment.
The problem is solved according to the apparatus by the fact that at least one agglomerator, with an inlet, an outlet each for the separated components (clear runnings, concentrated suspension) and an apparatus for producing the agglomeration, in particular an electric field, as well as a pressure filter with a feed inlet for the concentrated suspension and an outlet each for the separated components (clear runnings, filter cake), are provided.
The liquid to be cleaned is collected and agglomerated in the agglomerator with the apparatus for producing an electric field, for example at an electrode side. During the phase already cleaned liquid can be drawn off via the upper outlet. After disconnection of the electric field the agglomerated particles are suspended in the residual liquid once again and fed to the pressure filter as concentrated suspension via the second outlet. From there an outlet delivers the cleaned liquid. The filter cake can be extracted from the second outlet of the pressure filter as required.
It is preferred that a first collecting tank is provided downstream of the agglomerator in the inflow, in order to permit in the event of batch-wise operation of the agglomerator a liquid supply independent of the latter.
If a vacuum pump is connected to the first collecting tank, a low pressure of for example 40 to 120 mbar can be set in the collecting tank. For the further improvement of the adsorption of the fed bleaching earth it is also advantageous to heat the liquid in the tank to a temperature of 90xc2x0 C. to 130xc2x0 C.
If a second collecting tank is provided at the outlet of the agglomerator, whose outlet leads to the pressure filter via a pump, it is possible, in order to clean the pressure filter, for the supply of concentrated suspension to be interrupted for a short time without the operation of the agglomerator thereby being affected. The concentrated suspension is buffered in the second collecting tank.
Because at least two parallel connected agglomerators are provided, which are operated stepwise, the liquid to be cleaned can be supplied practically continuously, whereas the parallel connected agglomerators are operated stepwise or alternately. In this case the provision of a first collecting tank upstream of the agglomerators is not necessary.